Various systems are available and used to manufacture sheets of paper and other paper products. The sheets of paper being manufactured often have multiple characteristics that are monitored and controlled during the manufacturing process, such as dry weight, moisture, and caliper (thickness). The control of these or other sheet properties in a sheet-making machine is typically concerned with keeping the sheet properties as close as possible to target or desired values.
During the manufacturing process, it is common for a paper sheet being produced to tear or break. When this occurs, the paper sheet is typically rethreaded through the sheet-making machine, and operation of the sheet-making machine resumes. However, for a period of time after the rethreading, the paper sheet produced by the sheet-making machine is typically not usable or saleable. This is because the break in the paper sheet often disturbs or interferes with the control of the sheet-making machine, so the paper sheet produced after the break typically has sheet properties that are not near the target or desired values. As a result, the sheet-making machine often needs to be operated until the disturbances caused by the break are eliminated and the sheet properties return to or near the target or desired values. This typical results in a loss of both time and materials.
As a particular example, the caliper or thickness of a paper sheet is often controlled by passing the paper sheet between counter-rotating rolls. The space between two rolls is often referred to as a “nip.” The pressure applied by the rolls to the paper sheet is typically controlled by varying the temperature of the rolls. For example, heating the rolls typically causes the diameter of the rolls to expand, decreasing the size of the nip and increasing the pressure applied to the paper sheet. This compresses the paper sheet and reduces its thickness. By controlling the temperature of the rolls, the pressure applied by the rolls to the paper sheet may be controlled, thereby facilitating control over the paper sheet's thickness. However, if a break in the paper sheet occurs, the temperature of the rolls may change significantly. When the paper sheet is rethreaded in the sheet-making machine, the thickness of the paper sheet may be far from the target or desired caliper value.